Specifying hosts using a host list file

Each host is included on a separate line. For example, the following display the contents of a simple host list file called myhosts:

node1.mydomain.com
node2.mydomain.com
node3.mydomain.com
node4.mydomain.com

In a host list file, the order of the nodes in the file is not preserved when you launch processes across resources. In the previous example, the host list that is named myhosts and the node1.mydomain.com entry might not be the first node that is used, even though it is listed first in the host list file. For example, the following might be the order in which the nodes are used:

1. node3.mydomain.com
2. node2.mydomain.com
3. node1.mydomain.com
4. node4.mydomain.com

After you have created the host list file, you can specify it on the command line using the --hostfile (also known as --machinefile) option of the mpirun command. For example, using the simple myhosts host list file, you could run your application, prog01, as follows:

mpirun -np 4 --hostfile myhosts prog01

For each host, the host list file can also specify:

• The number of slots (the number of available processors on that host). The number of slots can be determined by the number of cores on the node or the number of processor sockets. If no slots are specified for a host, then the number of slots defaults to one. In this example, a host list file called myhosts specifies three nodes, and each node has two slots:

  cat myhosts
  node1 slots=2
  node2 slots=2
  node3 slots=2

  Specifying the following command launches six instances of prog01; two on node1, two on node2, and two on node3:

  mpirun -hostfile myhosts prog01

• The maximum number of slots. Note that the maximum slot count on a host defaults to infinite, thereby allowing IBM Spectrum® MPI to oversubscribe to it. To avoid oversubscribing, you can provide a maximum slot value for the host (max-slots=*).

The host list file can also contain comments, which are prefixed by a pound sign (#). Blank lines are ignored.

For example:

 
# This is a single processor node:
node1.mydomain.com
 
# This is a dual-processor node:
node2.mydomain.com slots=2
 
# This is a quad-processor node. Oversubscribing 
# to it is prevented by setting max-slots=4:
node3.mydomain.com slots=4 max-slots=4